1. Field of the Invention
The invention relates to a power supply device, more specifically, to a cooling control for the power supply device.
2. Description of the Related Art
Power storage bodies, such as battery cells or capacitors, in a power supply device generate heat at the time of charging/discharging. A power storage body, such as a fuel cell or a capacitor in a power supply device, generates heat at the time of charging/discharging. Therefore, by cooling the power storage bodies using a cooling device provided in the power supply device, the temperature of the entire power supply device is controlled to make the output from the power storage bodies constant, to extend the lifetime of the power storage bodies, and to supply constant electric power.
Examples of a method of cooling the power supply device (the power storage bodies) include a gaseous cooling method and a liquid cooling method. In these cooling methods, heat transferred from the power storage bodies to a gaseous or liquid cooling medium is transferred to a casing constituting a part of the power supply device, and then is discharged from the power supply device. The gaseous cooling medium used in the gaseous cooling method is easier to handle than the liquid cooling medium used in the liquid cooling method. However, the gaseous cooling medium has lower heat conductivity than that of the liquid cooling medium. In contrast, in the liquid cooling method, the liquid cooling medium needs to be carefully handled. For example, a sealing mechanism needs to be provided to prevent leaking of the cooling liquid from the power supply device. However, the liquid cooling medium cools the power supply device (the power storage bodies) more efficiently than the gaseous cooling medium, because the liquid cooling medium has significantly higher heat conductivity than that of the gaseous cooling medium.
In recent years, a power supply device, such as a secondary battery or an electric double-layer capacitor (condenser), has been employed as a battery for a hybrid vehicle and an electric vehicle. In such a power supply device, a plurality of power storage bodies are disposed close together to make the power supply device compact. Thus, the power supply device can output high electric power. Therefore, in most cases, the liquid cooling method is employed, and thus the liquid cooling medium having high heat conductivity is used so that the heat inside the power storage bodies disposed close together is efficiently discharged from the outer peripheries of the power storage bodies.
When the liquid cooling method is employed, the cooling liquid is filled in a casing that constitutes a part of the power supply device, and the plurality of power storage bodies are disposed in the casing in which the cooling liquid is filled. A lid member seals the cooling liquid and a power storage module including the plurality of power storage bodies, in the casing. When the power storage bodies generate heat due to charging/discharging, the heat is transferred to the cooling liquid, and then the heat is transferred from the cooling liquid to the casing. Then, the heat is discharged from the power supply device. At this time, convection (natural convection) of the cooling liquid occurs in the sealed casing, as in the case of gas. The heat generated in the power storage bodies is discharged from the power supply device due to the effect of the convection, and the heat conductivity of the cooling liquid.
As described above, the heat conductivity and the convection of the liquid are main factors for improving the cooling efficiency in the liquid cooling method. For example, as described in Japanese Patent Application Publication No. 2005-19134 (JP-A-2005-19134), the technology, in which a cooling liquid in a power supply device is forcibly circulated by a pump to improve cooling efficiency through the forced convention, has been proposed. Further, Japanese Patent No. 2775600 (JP-B-2775600) and Japanese Patent Application Publication No. 9-266016 (JP-A-9-266016) describe similar technologies.
In the power supply device in which the plurality of power storage bodies are provided, if performance varies among the power storage bodies, the lifetime of an entire power storage device is decreased. More specifically, if the temperature of the cooling liquid around the power storage bodies varies depending on the portion of the cooling liquid, the cooling liquid has a strong cooling effect on a part of the plurality of power storage bodies, and has a weak cooling effect on another part of the plurality of power storage bodies. Thus, the deterioration rate varies among the power storage bodies. This decreases the lifetime of the power supply device.
Accordingly, in the technologies described in the JP-A-2005-19134, the JP-B-2775600, and the JP-A-9-266016 as described above, a cooling liquid is forcibly circulated (such that the cooling liquid flows into a battery apparatus through an inlet, and discharged from the battery apparatus through an outlet). However, in the power supply device in which the plurality of power storage bodies are disposed close together, the cooling liquid is not sufficiently circulated among the power storage bodies disposed close together. Therefore, for example, the temperature of the cooling liquid in areas among the power storage bodies differs from the temperature of the cooling liquid at the outer peripheries of the power storage bodies, and thus the power storage bodies cannot be appropriately cooled. Therefore, the lifetime of the power supply device is decreased.